Cardiac assistance system provide additional cardiac output in patients who suffer from insufficient cardiac output. One type of cardiac assistance system is called a left ventricular assist device (LVAD). LVADs are auxiliary pouches intended to function as booster pumps to aid the hearts of individuals suffering from chronic congestive heart failure. This condition is frequently due to heart attacks that reduce the pumping capacity of the human heart. By boosting the capacity of such a weakened heart, individuals suffering from this condition may be allowed to again lead relatively normal, effective lives.
While various designs of LVADs have been proposed, the most promising appears to be an auxiliary pouch formed from the individual's latissimus dorsi muscle and controlled by a pacemaker. This approach avoids potential rejection problems related to the use of non-autologous materials and takes advantage of well developed pacemaker and prosthetic vascular graft technology. LVADs of this type are commonly called skeletal muscle ventricles (SMVs).
To create an SMV in a human involves exposing the left latissimus dorsi muscle and dissecting the muscle free from the subcutaneous tissues and chest wall, except for the neurovascular bundle and humeral insertion. A bipolar nerve cuff electrode is placed around the thoracodorsal nerve. The nerve lead is connected to an inactive neurostimulator, buried beneath the left rectus abdominis muscle, which innervates the exposed latissimus dorsi muscle.
Next, the left chest is opened at the fourth rib. Preferably, the fourth rib is removed to provide more space for the LVAD. Optionally, the anterior pericardium is removed between the phrenic nerves and used to cover a conically-shaped mandrel of biocompatible plastic. Mandrels used for beagles had a diameter of about 3 cm, length of about 6.5 cm and volume of about 25 ml; a mandrel suitable for forming a human SMV would need to be appropriately enlarged. After wrapping the pericardium around the mandrel it is sewn to a 5 mm thick collar of synthetic material such as woven DACRON felt placed at the base of the mandrel. The dorsal edge of the latissimus dorsi muscle is then folded longitudinally upon itself and secured by sutures, after which the medial aspect of the latissimus dorsi muscle is wrapped around the mandrel (and over the pericardium if it was used) about 2-2.5 times with the folded edge of the muscle sewn circumferentially to the Dacron felt collar. The SMV is then positioned DACRON and the wound is closed and allowed to heal for three weeks.
Following this healing period, a stimulator such as the Medtronic Model 4710 is activated to transform the fatigable Type II latissimus dorsi muscle fibers to fatigue-resistant Type I muscle fibers. Typically, 6 weeks are allowed for this stimulation period, after which the chest is again opened to connect the formed muscle pouch to the aorta. This is accomplished by first attaching sensing leads to the left ventricle. The descending thoracic aorta is exposed to allow two 12 mm ringed vascular grafts to be anastomosed to the aorta, one above the other, in end-to-side fashion.
After completion of these anastomoses, the aorta between the two graft anastomoses is at least partially ligated. The plastic mandrel is removed from within the muscle pouch and a sewing ring is used to couple the pouch to the aorta. The aorta is then at least partially ligated forcing blood flow through the newly formed SMV. Finally, the nerve lead and myocardial leads are connected to an R-wave synchronous pulse-train stimulator.
One problem such past cardiac assist systems have faced, however, is with the sewing ring. In particular, because the sewing ring is a foreign object to the body, the body's immune system often vigorously attacks it. A common result is that the object and the surrounding tissue develop a layer of fibrous tissue. Fibrous tissue around the sewing ring and within the skeletal muscle ventricle diminishes the ability of the SMV to pump blood. Another problem which may occur is the rupture of the SMV near the sewing ring. Such a rupture, in fact, is more likely to occur the more the sewing ring has been encapsulated by such fibrous tissue, as opposed to being well-incorporated by the skeletal muscle. Thus there exists a need for a sewing ring which will minimize the formation of fibrous tissue about the ring and in the surrounding SMV tissue.